Mathematics – Logic
Scientific paper
Jul 1993
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993metic..28..407n&link_type=abstract
Meteoritics, vol. 28, no. 3, volume 28, page 407
Mathematics
Logic
1
Alkali Elements, Alteration, Chondrules, Cv Chondrites, Rb-Sr Systematics
Scientific paper
In order to clarify when and where the distributions of alkalis in chondrules were established [1-3] we have undertaken a correlated study of Rb-Sr systematics and petrologic properties of chondrules from the Allende (CV) meteorite. We report here the results of Rb-Sr isotopic analyses combined with petrologic examinations for 16 chondrules (19 specimens) from Allende. The whole-rock analyses indicate a model age of 4.5 +/- 0.05 Ga (using ALL [4]), suggesting that the Rb-Sr isotopic system has been closed in bulk Allende for 4.5 yr. For the chondrules, the earlier works [4,5] include a few barred-olivine and many unknown petrographic types (probably mostly porphyritic) chondrules. In this work, we present results for 1 radial pyroxene, 4 porphyritic (7 specimens), and 11 barred olivine chondrules, covering major petrographic types. The results are shown in an 87Rb-87Sr evolution diagram (Fig. 1). Including earlier results [4,5], no systematic differences are found for different textural types, but data points mostly deviate from the 4.5-Ga line to the right, forming a rough linear array. This means that the Rb-Sr isotopic system in chondrules have been disturbed by a late event(s). The slope of the dotted line corresponds to an age of ~4.0 Ga. The meanlng of the linear trend is not clear because of the lack of isotopic equilibrium. Ground masses of chondrules analyzed for Rb-Sr isotopes were examined by optical microscope and EPMA. Some chondrules have primary glassy ground masses enriched in the anorthite component, especially in their central parts. However, as demonstrated previously [6], most of the chondrules analyzed here for Rb-Sr isotopes were also altered. They have abundant nepheline and sodalite components that replace primary glassy ground masses. Sodium was secondarily introduced and Ca was lost from the Allende chondrules. Using abundances of dodalite and nepheline components, the degree of alteration was tentatively classified into three categories: A (least altered), B (middle), and C (most altered). In Fig. 1, the chondrules with higher 87Rb/86Sr are mostly in categories of B or C. On the other hand, the chondrules of category A have the lower 87Rb/86Sr ratio, relatively close to that of bulk Allende. The 87Rb/86Sr ratios are related to the alteration categories but are not directly related to the isotopic deviation. It is thus considered that the general trend of high and low elemental ratios of Rb/Sr had been basically established during the early nebular processes and were then modified significantly more recently. The diffusivities of alkalis in glassy materials [7] at low temperatures (~400 degrees C) suggest a possible migration of Rb as well as Na from matrix to ground masses of chondrules. The old model age for Allende "matrix" [5] is consistent with such a possibility. We therefore strongly suggest that the Allende chondrules were subjected to a low- temperature alteration reaction after consolidation of the chondrules. References: [1] Grossman J. N. and Wasson J. T. (1983) In Chondrules and Their Origins (E. A. King, ed.), 88-121, LPI. [2] Hewins R. H. (1991) GCA, 55, 935-942. [3] Matsuda H. et al. (1990) Meteoritics, 25, 137-143. [4] Gray C. M. et al. (1973) Icarus, 20, 213-239. [5] Tatsumoto M. et al. (1976) GCA, 40, 617-634. [6] Kimura M. and Ikeda Y. (1992) Papers 17th Symp. Antarct. Meteor., 31-33. [7] Jambon A. and Carron J. P. (1976) GCA, 40, 897-903. Fig. 1, which appears here in the hard copy, shows an 87Rb-87Sr evolution diagram for chondrules from the Allende meteorite.
Kimura Masahiro
Nakamura Nobuhiro
Nohda Susumu
Shimoda Haruhisa
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